For me it's simply 50/50 Pilsner malt & wheat malt. Mash at 66C for 60min. Standard mash out, nothing special there.
Transfer to your boil kettle (or lift out the grain if you're doing BIAB, just follow your standard ale/lager method). Boil for 15min, chill to 40C, pitch 1 capsule of Essential Nutrients IBS Support per 20L of wort. Cover the kettle opening with glad wrap and sit the lid on top to form a proper seal. Maintain @ 40C or at least over 30C.
3 days (72hrs) later, re-boil to kill the lacto bugs and add a small handful of hops, to no more than 5IBU. This boil should be 45min in length, meaning the full boil length of the recipe was 60min (15min to kill lacto, 45min to do hop additions / encourage evaporation).
Chill to normal ale fermentation temp and ferment as you would a normal ale. Pitch whatever ale yeast you desire, ensure it's a big pitch since the lower pH may inhibit yeast activity.
My watermelon sour contained 2.5L of strained watermelon flesh puree and I swear to you it was feggin' magical.